1. Field of the Invention
The present invention relates to a laser beam-combining optical device, and more particularly to a laser beam-combining optical device for combining a plurality of output light beams output from a multi-emitter laser having a plurality of light-emitting point sources.
2. Description of the Background Art
Conventionally, a projection-type display device such as a projector uses a high-pressure mercury lamp or the like as the light source. The emission spectrum of a lamp includes generally continuous visible light range depending on properties of a substance contained in the lamp. In order to create an optimum color for the display device, the emission spectrum of a lamp is separated into three colors: red, green, and blue. Small display devices are then irradiated by light of the respective colors, and light rays of the respective colors are combined together again on the screen. Thus, full-colored video is generated.
In recent years, projection-type display devices that use lasers as the light sources, instead of lamps such as high-pressure mercury lamps, have been actively studied. A projection-type display device that uses a laser as the light source uses monochromatic lasers as the light source, and therefore provides a wide range of color reproduction. Moreover, since laser light originates from a small light-emitting point source, the size of an entire optical system can be reduced. Furthermore, a red, green, and blue monochromatic light rays, for example, are emitted from a red, green, and blue lasers, respectively, and therefore there is no need for an optical system for separating colors in contrast to one that uses a lamp as the light source.
However, if only a single laser element is used as the light source of a projection-type display device, current technology cannot provide sufficient output power of the laser element. In particular, use of a projection-type display device in an application that involves projection of image light onto a large screen cannot provide a sufficient amount of light. Thus, means configured to include a plurality of single-emitter lasers each having a single light-emitting point source, and combine the output thereof to provide high output power is proposed, for example, in Japanese Patent Application Laid-Open No. 2000-241659.
Japanese Patent Application Laid-Open No. 2002-202442 proposes a method in which light rays from a single multi-emitter laser (also referred to as “semiconductor laser array”) having a plurality of light-emitting point sources are collected and coupled into optical fiber.
Also, Japanese Patent Application Laid-Open No. 2007-17925 proposes a technique in which light beams from a plurality of lasers are aligned using polarization members, and are then collected and directed into optical fiber using a condenser lens.
However, Japanese Patent Application Laid-Open No. 2000-241659 described above provides different path lengths of laser light beams from respective laser elements to a condenser lens, and thus the radii of the light beams at the location of the condenser lens differ from one another. Therefore, it is difficult to obtain a sufficiently small focus point.
Japanese Patent Application Laid-Open No. 2002-202442 describes a configuration such that light rays from a single multi-emitter laser are collected. This configuration, however, creates difficulties in combining outputs of a plurality of multi-emitter lasers.
The light-collecting optical system of Japanese Patent Application Laid-Open No. 2007-17925 only has a function to combine light waves, and it is thus difficult to provide aberration correction, and also to reduce the radius of focus spot. In addition, appropriate aberration correction requires high accuracy in the shape and arrangement of the collimate lens. This presents a problem both in cost and in productivity.